Recent years have seen the development of diagnostic techniques that allow the practicing clinician to obtain high fidelity views of the anatomical structure of the human body. Imaging systems such as computed tomographic (CT) x-ray imagers, positron emission tomographic (PET) scanners, single photon emission computed tomography (SPECT) scanners and nuclear magnetic resonance imaging (MRI) machines have provided clinicians with the ability to improve visualization of the anatomical structure of the human body without surgery or other invasive techniques. In lieu of exploratory surgery, the patient can be subjected to the scanning modalities of such imaging systems, and the patient's anatomical structure can be reproduced in a form for evaluation by a trained doctor.
The doctor sufficiently experienced in these techniques can evaluate the images of the patient's anatomy and determine if there are any abnormalities present. An abnormality in the form of a lesion appears on the image as a shape that has a discernable contrast with the surrounding area. The difference in contrast is due to the lesion having imaging properties that differ from those of the surrounding body tissue. Moreover, the contrasting shape that represents the lesion appears at a location on the image where such a shape would not normally appear with regard to a similar image of a healthy person.
Once a lesion has been identified, several methods of treatment are utilized to remove or destroy the lesion, including chemotherapy, radiation therapy, and surgery. When chemotherapy is chosen, drugs are introduced into the patient's body to destroy the lesion. During the course of treatment, imagers are commonly used to follow the progress of treatment by subjecting the patient to periodic scans and comparing the images taken over the course of the treatment to ascertain any changes in the lesion configurations.
In radiation therapy, the images of the lesion generated by the imager are used by a radiologist to adjust the irradiating device and to direct radiation solely at the lesion while minimizing or eliminating adverse effects to surrounding healthy tissue. During the course of the radiation treatment, the imaging system is also used to follow the progress of the patient in the same manner described above with respect to chemotherapy.
When surgery is used to remove a lesion or other abnormality, the images of the lesion in the patient can guide the surgeon during the operation. By reviewing the images prior to surgery, the surgeon can decide the best strategy for reaching and biopsying, excising, or otherwise manipulating the abnormality or lesion, whether it is a brain tumor, arteriovenous malformation, infection or other entity. After surgery has been performed, further scanning is utilized to evaluate the success of the surgery and the subsequent progress of the patient.
A problem associated with the scanning techniques mentioned above concerns the accurate selection and comparison of views of identical areas in images that have been obtained by imagers at different times or by images obtained essentially at the same time using different image modalities, e.g., CT, MRI, SPECT, and PET. This problem has two aspects. First, in order to relate the information in an image of the anatomy to the anatomy itself, it is necessary to establish a one-to-one mapping between points in the image and points on the anatomy. This is referred to as registering image space to physical space.
The second aspect concerns the registration of one image space to another image space. The goal of registering two arbitrarily oriented three dimensional images is to align the coordinate systems of the two images such that any given point in the scanned anatomy is assigned identical addresses in both images. The calculation of the rigid body transformation necessary to register the two coordinate systems requires knowledge of the coordinate vectors of at least three points in the two systems. Such points are called "fiducial points" or "fiducials," and the fiducials used are the geometric centers of markers, which are called "fiducial markers". These fiducials are used to correlate image space to physical space and to correlate one image space to another image space. The fiducial markers provide a constant frame of reference visible in a given imaging mode to make registration possible. The general technique for using fiducial markers to obtain registration of image data across time is set forth in U.S. Pat. No. 4,991,579 to George S. Allen, the contents of which are incorporated herein by reference.
One problem extant in the field lies in the provision of fiducials capable of use with several imaging modalities. MRI and X-ray CT images are digital images, in which the images are formed point by point. These points are called picture elements, or pixels, and are associated with an intensity of light emitted from a cathode ray tube, or are used to form an image on film. The array of lighted pixels enables the observer to view an image. The manner in which the intensity of any given pixel is altered or modulated varies with the imaging modality employed. In X-ray CT, such modulation is a function primarily of the number of electrons per unit volume being scanned. In MR imaging, the parameters primarily influencing this modulation are the proton spin density and longitudinal and transverse relaxation times T1 and T2, which are also known as the spin-lattice and spin-spin relaxation times, respectively. In constructing a fiducial marker, one must be aware that an agent that can be imaged under one imaging modality will not necessarily be imageable under another modality. And yet, the ability to image under both CT and MRI with a given marker would be especially useful, in that one would then be able to register images derived from different imaging modalities. For example, the capability to register CT and MR images would allow the integration of information concerning bony structure provided by a CT scan with the soft tissue anatomical information provided by an MRI scan. There remains a need for a fiducial marker that can be used to establish a known coordinate system under several imaging modalities.
A further problem in the field arises from the competing needs of accommodating patient comfort, which would tend to lead clinicians toward the minimization of marker size, with the desire of clinicians to use markers that are as bright and thus as large as possible. Such brightness is desirable because it provides a strong signal that can be distinguished from noise inherent in the imaging process. The use of large-sized markers is also desirable so that the image of the marker occupies as many pixels as possible. Increasing the number of pixels occupied by the marker increases the accuracy with which the position of the marker can be determined. Furthermore, the general technique of using fiducial markers requires the determination of the centroid of the marker; it is easier to compute the centroid for a large, bright marker than for a smaller, dimmer marker. On the other hand, the larger the marker is, the more difficult it is for the patient to tolerate its presence for extended periods of time. There remains a need for a marker which can exploit the advantages presented by increased size that would also be tolerated by the patient during the period of its use. There is also a need for a small multi-modality marker that can be implanted into a patient and remain there for more extended periods of time. Such a more permanent fiducial marker would preferably be detectable by a non-invasive technique so that its position in physical space could be determined and its centroid computed even as it remained hidden from visual inspection beneath the patient's skin.